ESP8266 Community Forum

- Thu Jan 04, 2018 10:36 am #72886 Hello all,

I have a project where I am using an ESP8266 to take temperature of 4 DHT22 sensors. These connected to the ESP on a CAT5 cable with a length of around 60 feet with the sensors being placed along the wire.

My question is: would it be more power efficient to power the DHT22 with one of the ESP output pins, or would it make more sense to have one of the ESP pins control a MOSFET or transistor to switch on the supply voltage to the DHT22 when taking a reading. I am trying to save as much power as possible.
From the datasheet it appears the pins of the ESP8266 can supply up to 12ma, but I am not sure how the long cable run will effect it?

Finally, does the mosfet or transistor use less current when operating?

Thanks!

- Mon Jan 08, 2018 5:20 am #72962 I think it would be OK to power 4 of those direct from an ouput pin. The load would be fairly low, particularly with each measuring separately.

I would put the 100nF on the Vdd near each sensor and maybe include a small serial resistor (e.g. 100 Ohm) to limit the pulse current.

In general I use mosfets rather than bjts for switching. They are almost perfect switches (low on resistance, no saturation voltage, high current capability) and they are cheap. The gate drive means no need to worry about supplying enough base current. One just has to ensure that you use low gate threshold compatible with 3.3V logic. My preferred device at the moment is the AO3400 which can switch up to a couple of amps.

- Mon Jan 08, 2018 8:27 am #72967 I do not think it will matter how you feed it. With this length of cable it does will be not work in my opinion.

- Mon Jan 08, 2018 1:19 pm #72972

I do not think it will matter how you feed it. With this length of cable it does will be not work in my opinion.

I tend to agree. I would think it would work under ideal conditions but I would never design a real product like that. The length may mess up the pulse signals. But I would also be concerned about transient noise damaging the devices. If I were to do this I would include some ESD protection.